Object entries in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does Object.entries() do in JavaScript?

Object.entries() takes an object and returns an array of its own enumerable property [key, value] pairs.

Think of it like turning a box of labeled items into a list of label-item pairs.

beginner

How is the output of Object.entries() structured?

The output is an array where each element is a two-item array: the first item is the property key (a string), and the second is the property value.

Example: [['name', 'Alice'], ['age', 30]]

intermediate

Can Object.entries() be used on arrays? What happens?

Yes! Arrays are objects with numeric keys as strings. Object.entries() returns pairs of index and value.

Example: Object.entries(['a', 'b']) returns [['0', 'a'], ['1', 'b']].

intermediate

Does Object.entries() include inherited properties?

No. It only includes the object's own enumerable properties, not those inherited from its prototype.

beginner

How can Object.entries() help in looping over an object?

<p>You can use <code>Object.entries()</code> with <code>for...of</code> to get key and value in each loop step.</p><p>Example:<br><code>for (const [key, value] of Object.entries(obj)) { console.log(key, value); }</code></p>

What does Object.entries({a: 1, b: 2}) return?

A {a: 1, b: 2}

B ['a', 'b']

C [1, 2]

D [['a', 1], ['b', 2]]

Object.entries() returns an array of [key, value] pairs, so the correct answer is [['a', 1], ['b', 2]].

Does Object.entries() include properties from the object's prototype?

A No, only own enumerable properties

B Yes, all properties

C Only non-enumerable properties

D Only properties starting with underscore

Object.entries() only includes the object's own enumerable properties, not inherited ones.

What is the type of each element inside the array returned by Object.entries()?

A Array of two elements

B String

C Number

D Object

Each element is an array with two items: the key and the value.

What will Object.entries(['x', 'y']) return?

A [0, 1]

B [['0', 'x'], ['1', 'y']]

C ['x', 'y']

D []

Arrays are objects with numeric keys as strings, so Object.entries() returns index-value pairs.

Which loop works well with Object.entries() to access keys and values?

A for...in

B while

C for...of

D do...while

for...of loops over iterable objects like arrays, making it perfect for Object.entries() output.

Explain how Object.entries() transforms an object and give a simple example.

Think about turning a labeled box into a list of label-item pairs.

Describe how you can use Object.entries() to loop through an object's properties.

Focus on how to get both key and value easily in a loop.

Practice

1. What does Object.entries(obj) return when used on an object obj?

easy

A. A new object with the same keys and values

B. An array of values only

C. An array of keys only

D. An array of [key, value] pairs from the object

Solution

1. Step 1: Understand Object.entries behavior

   Object.entries takes an object and returns an array where each element is a two-item array: the key and its corresponding value.

2. Step 2: Compare options to definition

   Only An array of [key, value] pairs from the object correctly describes this output as an array of [key, value] pairs. Options A, C, and D describe different outputs.

3. Final Answer:

   An array of [key, value] pairs from the object -> Option D

4. Quick Check:

   Object.entries = array of [key, value] pairs [OK]

Hint: Remember: entries give key-value pairs as arrays [OK]

Common Mistakes:

• Thinking it returns an object
• Confusing entries with keys or values only
• Expecting a flat array instead of pairs

2. Which of the following is the correct syntax to get entries from an object data?

easy

A. entries(Object.data)

B. Object.entries(data)

C. Object.entry(data)

D. data.entries()

Solution

1. Step 1: Recall the correct method name and usage

   The correct method is Object.entries() called with the object as argument.

2. Step 2: Check each option

   Object.entries(data) uses the correct syntax. data.entries() is invalid because objects don't have an entries() method. Object.entry(data) uses a wrong method name. entries(Object.data) has incorrect syntax.

3. Final Answer:

   Object.entries(data) -> Option B

4. Quick Check:

   Correct method is Object.entries(obj) [OK]

Hint: Use Object.entries(obj), not obj.entries() [OK]

Common Mistakes:

• Calling entries as a method on the object
• Misspelling 'entries' as 'entry'
• Passing object incorrectly

3. What is the output of this code?

const obj = {a: 1, b: 2};
console.log(Object.entries(obj));

medium

A. ['a', 'b']

B. [['1', 'a'], ['2', 'b']]

C. [['a', 1], ['b', 2]]

D. [1, 2]

Solution

1. Step 1: Apply Object.entries to the object

   Object.entries returns an array of arrays, each inner array has the key and value from the object.

2. Step 2: Match output to options

   For obj = {a:1, b:2}, output is [['a', 1], ['b', 2]] which matches [['a', 1], ['b', 2]] exactly.

3. Final Answer:

   [['a', 1], ['b', 2]] -> Option C

4. Quick Check:

   Object.entries(obj) = [['a', 1], ['b', 2]] [OK]

Hint: Entries output is array of [key, value] pairs [OK]

Common Mistakes:

• Swapping keys and values
• Expecting a flat array of keys or values
• Confusing output format with JSON string

4. Find the error in this code snippet:

const user = {name: 'Alice', age: 25};
for (const [key, value] of user.entries()) {
  console.log(key, value);
}

medium

A. user.entries() is not a function

B. Missing curly braces in for loop

C. Destructuring syntax is incorrect

D. Console.log syntax error

Solution

1. Step 1: Identify method usage on object

   Plain objects do not have an entries() method; Object.entries() is a static method on Object.

2. Step 2: Correct usage

   To iterate entries, use for (const [key, value] of Object.entries(user)) instead of user.entries().

3. Final Answer:

   user.entries() is not a function -> Option A

4. Quick Check:

   Objects lack entries() method; use Object.entries(obj) [OK]

Hint: Use Object.entries(obj), not obj.entries() [OK]

Common Mistakes:

• Calling entries() directly on object
• Confusing Object.entries with instance method
• Ignoring error message about missing function

5. Given const data = {x: 10, y: 0, z: null, w: 'hello'}, which code snippet correctly creates a new object with only entries having truthy values?

hard

A. Object.fromEntries(Object.entries(data).filter(([k, v]) => v))

B. Object.entries(data).filter(([k, v]) => v)

C. Object.entries(data).map(([k, v]) => v && [k, v])

D. Object.fromEntries(data.filter(([k, v]) => v))

Solution

1. Step 1: Filter entries with truthy values

   Use Object.entries(data) to get pairs, then filter pairs where value is truthy (non-zero, non-null, non-false).

2. Step 2: Convert filtered entries back to object

   Use Object.fromEntries() on filtered array to create new object with only truthy values.

3. Final Answer:

   Object.fromEntries(Object.entries(data).filter(([k, v]) => v)) -> Option A

4. Quick Check:

   Filter entries then fromEntries to object [OK]

Hint: Filter entries then use Object.fromEntries to rebuild object [OK]

Common Mistakes:

• Forgetting to convert back to object
• Using map instead of filter
• Trying to call filter on object directly